The reason for not having preconceived ideas about the data is to see what the data actually says. As Urquhart puts it:

If we privilege other theories rather than looking at the data, we lose what is for me the key delight - and the key edge of the method: what Glaser one of GTM's founders calls 'emergence'. The idea of emergence, for me, is that we stay true to our data, we look for what the data is telling us.

It sounds good in theory, but I'm still trying to grapple with exactly how this works.